passive metal

Topics: Corrosion, Aluminium, Metal Pages: 1 (336 words) Published: October 22, 2014
Passive metal is a metal on which a protective film that prevents further attack on the metal is readily formed by natural process or by immersion in a passivating solution. The process is called passivation. The passivation process returns the stainless steel or other metals back to its original specifications by removing unwanted debris and oils from the surface and then submerging the part into a passivating bath. The surface film created can cause the surface to lose its chemical reactivity and being less affected by environmental factors. The process improves and purifies the surface of the part. The restored surface acts like a protective coating to environmental factors such as air, water and other extreme environments thus significantly reduce the corrosion rate of the metal. The typical films produced are oxides, hydroxides and sulphates. Some examples of metals that exhibit passivity are iron, chromium, titanium, nickel and alloys containing these metals.

The corrosion products are the substances produced during corrosion reaction. The product can be soluble and insoluble. The insoluble product produces often act as a protecting film which greatly reduced the corrosion rate of the metal. The tendency to form an insoluble film is expressed as a solubility product which defines the concentration of dissolved metal ions. As the metal ions concentration and the hydroxyl/oxide/sulphates ions increase, the likelihood of formation of passive film increases. The anode is where generally the corrosions occur, where the metals atoms go into solutions as metal ions. Since the corrosion reactions involve the transfer of electrons and ions between the metal and the solution, the rates are equivalent to electric currents. The rates of these reactions depend on the potential difference between the metal and the solution, i.e. the potential of the metal.

Resistance polarization causes potential of anode and cathode to differ due to potential drop across...
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